I can't believe no one mentioned Red Dead Redemption. They let you get in fist fights with with pretty much anyone in town. You could get some amazing fights going on in that game. Just piss a few dudes off, climb a tall building, and then fist fight on top of it. When people went unconscious they would rag doll and fall off buildings and the like. Easily one of the most fun things to do in that game. It's more fun to go 3 rounds with some random dude than kill half a dozen guards.
Could work like in Read Dead exept when people wake up from unconsuiness they wont restart the fight, and they would be knocked out longer. Also, if you continued to hit them they could die. Maybe even an option to mount them while their downed and get in some ground and pound :toughninja: